The present invention relates to a novel estradiol derivative-alkylating agent conjugate with reduced hormonal activity, a process for preparing the conjugate, compounds useful for the preparation of the conjugate, and a growth inhibiting composition containing the conjugate or estradiol derivative.
Among the conventional alkylating agents, although having a strong antitumor activity, there are many agents which fail to display their full medicinal effects. The primary reason is their limited dosage due to their undesirable side effects in medicinal use. A solution to this problem is to combine the alkylating agent with a carrier having a specific affinity for the site of tumor to form an alkylating agent-carrier conjugate. It is intended to let the alkylating agent accumulate specifically at the site of tumor so that the agent will exhibit its antitumor activity effectively while suppressing the occurrence of the undesirable side effects.
Based on this conception, a proposal has been made on preparation of an estradiol-chlorambucil conjugate using estradiol as carrier and an antitumor agent containing the conjugate as principal constituent (Japanese Patent Publication (KOKOKU) No. 58-10397 (1983)). This antitumor agent specifically accumulates at the site of tumor and exhibits a strong antitumor activity. Further, it gives almost no significant influence to the normal cells.
However, an antitumor agent is usually required to be administered over a long period of time, so that even a slight side effect which is quite insignificant in short-term administration may give rise to a serious problem in long-time administration of the such agent. Especially, accumulation of the slight side effect may be remarkable as the physical strength of the cancer patient is usually weakened.
In the case of the estradiol-chlorambucil conjugate, in long-time administration thereof, there is observed in some cases development of the same symptoms as seen in administration of estrogen.
The studies by the present inventors have disclosed that development of such symptoms is due to the action of estrogen which is released in small quantities from the estradiol-chlorambucil conjugate in the patient's body. It is thus considered that in long-time administration of this conjugate, estrogen released in small quantities therefrom is accumulated in some cases to such an extent as to induce its side effects.
Further, the present inventors have found that when a specific substitutent, for example methyl group, is introduced into the estradiol ring, the estrogen action is remarkably diminished without impairing the selective physiological activity (growth-inhibiting action) which is inherently possessed by the estradiol-alkylating agent. It was also found that estradiol derivatives usable for the preparation of the conjugate are very low in estrogen action although they have a growth inhibiting activity. The present invention has been attained on the basis of these findings.